The present invention relates to tires. More particularly, it relates to a tire comprising a specific arrangement of the architectural elements arranged in the sidewall zone, allowing the obtainment, on the one hand, under substantially normal pressure, of flexible sidewalls capable of imparting particularly favourable qualities, in particular of comfort and rolling resistance, and, on the other hand, under reduced pressure, of rigidified sidewalls capable of bearing the load on the tire, within certain limits.
For some years, tire manufacturers have been devoting considerable effort to developing original solutions to a problem dating back to the very first time use was made of wheels fitted with tires of the inflated type, namely how to allow the vehicle to continue on its journey despite a considerable or total loss of pressure in one or more tires. For decades, the spare wheel was considered to be the sole, universal solution. Then, more recently, considerable advantages have become apparent which may lead to the elimination thereof. The concept of “extended mobility” is being developed. The associated techniques allow travel to continue with the same tire, within certain limits, after a puncture or a drop in pressure. This allows the driver to travel to a repair point, for example, without having to stop, often in dangerous circumstances, to fit the spare wheel.
Two major types of extended mobility technology are currently available on the automobile market. On the one hand, there are tires of the self-supporting type (often known by their English abbreviation ZP, standing for “zero pressure”). Self-supporting tires are capable of bearing a load under reduced pressure, or indeed without pressure, thanks to sidewalls which are reinforced, most frequently by means of inserts of rubber material provided in the sidewalls. The structural rigidity of the sidewalls of a tire of this type is very high. Reinforced sidewall technologies favour operation in degraded mode, normally exceptional or at the very least very occasional for the vast majority of vehicles, to the detriment of everyday operation, which suffers the drawbacks intrinsic to the principle of reinforced sidewalls. In normal operation, at rated service pressure, this may entail considerable disadvantages in terms of rolling resistance and comfort. Furthermore, the strong tendency for the bottom zone of the tire to want to slide out of the rim under the effect of sidewall sagging may limit the impact of this solution.
Documents U.S. Pat. No. 6,453,961 and PCT/US99/11081 show various techniques used in tires with self-supporting sidewalls in order to reduce the impact of excessively rigid sidewalls in normal pressure operating mode. The first document describes a tire comprising a circumferential insert consisting of porous elastomeric material, situated in the axially inner portion of a sidewall insert. The porous portion makes it possible to maintain a degree of flexibility in the sidewall in normal mode. Under reduced pressure, the pores are squashed, thereby increasing the rigidity of the sidewall to allow good support. The second document describes a tire, of which one of the series of radial sidewall reinforcements consists of cords of variable modulus. A similar effect of varying the rigidity as a function of crushing the sidewall is obtained.
On the other hand, wheels are available which are equipped with supports capable of supporting the inside of the tread of a tire in the event of sagging of the sidewalls following a drop in pressure. This solution is advantageously combined with a tire comprising a bottom zone capable of minimising the risk of the tire sliding out of the rim. This solution is advantageous since it makes it possible to keep substantially intact the characteristics of travel under normal conditions. On the other hand, it exhibits the drawback of requiring an additional component, the support, for each of the wheels of the vehicle.
Thus, in order to overcome these various drawbacks, the invention provides a tire comprising at least one carcass-type reinforcement structure anchored on each side of the tire in a bead whose base is intended to be fitted on a rim seat, each bead being extended radially towards the outside by a sidewall, the sidewalls meeting up radially towards the outside with a tread and comprising at least one sidewall support means exhibiting a substantially progressive increase in rigidity, disposed in the substantially median portion of said sidewalls, the carcass-type reinforcement structure extending circumferentially from the bead towards said sidewall, a crown reinforcement, each of the beads comprising furthermore an anchoring zone allowing the reinforcement structure to be retained in each of said beads, said support means comprising at least one circumferential cord arranged on the circumference in different axial positions, so as to form around the circumference a succession of substantially regular undulations forming an undulating circumferential profile.
The solution proposed by the present invention makes it possible to eliminate the majority of the drawbacks associated with the current techniques allowing extended mobility. On the one hand, the tire does not require any support for travel at low pressure; on the other hand, it does not have rigid sidewalls reinforced for example by means of inserts of rubber material. The sidewalls exhibit a structural rigidity which varies with the deflection of the tire. Thus, when the tire travels at its rated pressure, the structural rigidity is that of a standard tire. The sidewalls exhibit a flexibility which is advantageously comparable to a conventional tire. Favourable characteristics such as a high level of comfort, a low rolling resistance, good endurance, etc., may be retained. The tire according to the invention thus exhibits the advantages associated with a self-supporting tire, which make it possible to ensure extended mobility, but without exhibiting the drawbacks which affect tire qualities during everyday use substantially at normal pressure.
In “extended mobility” mode, when the pressure drops the reduction in pressure causes an increase in the deflection and, by this very fact, the structural rigidity of the sidewalls: substantially progressively, in particular at the level of the zone of the tire in the vicinity of the area of contact with the ground, the rigidity of said support means increases with the progressive appearance of an increasing tensile force in the support means. These phenomena lead to a gradual blocking of the buckling of the sidewalls, until a supporting force is obtained which is capable of bearing the stresses transmitted by the sidewalls. Finally, after tensioning of the support means, sidewalls are obtained which are capable of bearing the load. The load is thus to a great extent borne by the sidewall support means. In contrast to the means of known type, these means, thanks to a circumferential-type action, produce a reaction of radial type.
Since, after stretching of the support means in the sidewalls, buckling of the sidewalls is in some way “blocked”, considerable, or indeed total, sagging of the tire does not take place, in particular at the level of the contact area. The load is thus to a great extent borne by the tensioning of the support means. Thus, with greatly reduced or zero pressure, the tire operates with greater deflection and a considerably higher structural rigidity, not only in order to prevent total sagging of the sidewalls but also to make it possible to bear the load despite the drop in pressure, in a manner comparable to a self-supporting tire.
Said cord(s) are arranged on the circumference in different axial positions, so as to form around the circumference a succession of substantially regular undulations forming an undulating circumferential profile. This is a clever and practical way of achieving the resilient function of the circumferential cords. In such a scenario, the cords may in themselves be substantially rigid. The required resilience is obtained because the cords are arranged in undulating manner.